1. Field of the Invention
The present invention generally relates to a navigation apparatus and a navigation method for a movable body, and more particularly to a navigation apparatus and a navigation method for a vehicle, which can display information related to a target position and a present position.
2. Description of the Related Art
There are navigation apparatus of so called self sustaining type (i.e. built in type) and navigation apparatus of GPS (Global Positioning System) type using artificial satellites. The navigation apparatus of self sustaining type is an apparatus for obtaining two dimensional displacement data of the present position of the movable body by earth magnetism data from an earth magnetism sensor, an azimuth data from an angular velocity sensor and a travelling distance data from a travelling distance sensor, and for integrating this two dimensional displacement data with respect to a standard point so as to measure a new present position of the movable body. The navigation apparatus of GPS type is an apparatus for receiving electric waves from more than two GPS satellites by a GPS receiver, and for calculating the present position of the movable body at the receiving point by pseudo distance data, which includes the time offset of the receiver between each GPS satellite and the receiving point, and positional data of each GPS satellite.
In the image display related to the navigation apparatus of a vehicle etc., a present position of the movable body, which indicates a travelling position of the movable body obtained by the above mentioned various position measurement data, and a target position, which indicates a target place set by an operator beforehand, are displayed on a map. Especially, when the travelling position of the movable body approaches the target place, both of the above mentioned movable body position and the target position are simultaneously displayed on one display picture plane.
In the above explained case, when the movable body position and the target position are displayed on one picture plane, an image display is performed as shown in FIG. 4A-4B.
In FIG. 4A, a position P, which indicates a movable body position, a position Q, which indicates a target position, and a position A, which is a center position of the display image, are indicated on the display picture plane. Dref is a distance of a reference radius expressed by dots (i.e. picture elements). Since the image on the display picture plane is formed by the set of dots, the dot can be used as a unit replacing the distance on the display picture plane. In FIG. 4A, the display picture plane is shown in the case where the movable body approaches the target place, and both of the movable body position P and the target position Q come into the display picture plane G.
In this manner, when both of the movable body position P and the target position Q, exist on the same display picture plane, the center position A of the display picture plane is set such that the area in front of the advancing direction of the movable body, is displayed more in area than the area at the back thereof. Namely, so called front wide concept is employed in this case. This is accomplished by setting the display coordinates of the map information to get such a relationship that the display position of the movable body position P is always on the circumference of a circle having the radius equal to the distance Dref around the center position A.
However, in the above mentioned navigation apparatus, when the distance between the movable body position P and the target position Q becomes less than the distance Dref as the movable body approaches the target place, the display positions of the movable body position P and the target position Q on the image display, are shifted from the center position of the display picture plane, so that the balance on the sight becomes bad as shown in FIG. 4B, which is the problem.
The cause of this problem is that the movable body position P on the display is always separated from the center position by the distance Dref according to the above mentioned condition, even if the movable body position P approaches the target position Q as close as possible. Namely, as the movable body approaches the target position, the display position of the movable body position P is not moved on the sight, instead, it is recognized on the sight that the target position Q approaches the movable body position P and that those two displays are finally collected at the position apart form the center position by the distance Dref.
On the other hand, there is another problem as following. Namely, the display position is frequently moved due to the measurement accuracy and error. Especially, at the vicinity of the target position Q, even under the condition that the movable body position P should still be displayed on the display, since the movable body is actually stopped, the movable body position P frequently moves (i.e. blurring) around the target position Q each time when this movement due to the measurement accuracy and error occurs. Consequently, the positional relationship between the movable body position P and the target position Q is difficult to be specified. This phenomenon is especially outstanding in case of the GPS measurement, when one GPS satellite, which has been used, is exchanged to another GPS satellite, since one GPS satellite cannot be used due to the degradation in the receiving condition.
FIG. 5, shows an example of such a blur caused by the measuring error of the GPS. In FIG. 5, the phenomenon is explained in which the navigation apparatus exchanges the reception of the electric wave from the GPS satellite and performs the measurement under the condition that the movable body is still. If the measurement is performed by exchanging the satellites between S1 to S4 among a plurality of GPS satellites which are the objects of signal reception and which conditions of signal reception are changed, the measurement result is changed each time the GPS satellite is exchanged. Accordingly, if this result is reflected to the image display, the display positions of the movable body and the target are not settled but blurred as indicated by P1 to P4, and Q1 to Q4 in FIG. 5.
In this manner, when the blurring phenomenon occurs due to the measurement error, the target position as well as the present position is difficult to recognize on the display picture plane of the display device.